In Linux user management, there's an important distinction between primary and secondary group memberships that often causes confusion. The primary group is assigned directly in the /etc/passwd file, while secondary groups are listed in /etc/group.

# /etc/passwd entry showing primary group (GID 12345)
filesender_1:x:1515:12345:filesender_1:/local/home/filesender_1:/bin/sh

# /etc/group entry showing only secondary members
valid_senders:x:12345:production_1

The system doesn't redundantly list primary group members in /etc/group because:

• The primary group relationship is already established in /etc/passwd
• This prevents circular references and maintains data consistency
• Group file only needs to track additional memberships

To properly check all group memberships (primary + secondary):

# Method 1: Using id command
id filesender_1
uid=1515(filesender_1) gid=12345(valid_senders) groups=12345(valid_senders)

# Method 2: Using getent
getent passwd filesender_1 | cut -d: -f4  # Shows primary GID
getent group valid_senders                # Shows secondary members

This behavior affects:

• File permissions (new files inherit primary group)
• Process privileges (both primary and secondary groups apply)
• Authentication systems (PAM modules check both)

Example of file permission impact:

# As filesender_1
touch testfile
ls -l testfile
-rw-r--r-- 1 filesender_1 valid_senders 0 Aug 1 10:00 testfile

For comprehensive group queries, consider these Bash functions:

# Check if user belongs to group (primary or secondary)
is_member() {
  user=$1
  group=$2
  gid=$(getent group "$group" | cut -d: -f3)
  [[ $(id -G "$user" | grep -w "$gid") ]] && return 0 || return 1
}

# List all groups for user including primary
all_groups() {
  user=$1
  primary_gid=$(id -g "$user")
  primary_group=$(getent group "$primary_gid" | cut -d: -f1)
  echo "Primary: $primary_group"
  id -Gn "$user"
}

When examining the /etc/group file, I noticed user filesender_1 isn't listed under group valid_senders:

valid_senders:x:12345:production_1

Yet, in /etc/passwd, the same GID (12345) appears as the primary group for filesender_1:

filesender_1:x:1515:12345:filesender_1:/local/home/filesender_1:/bin/sh

This isn't a discrepancy - it's how Linux/Unix systems fundamentally work:

1. Primary Group: Assigned in /etc/passwd (4th field)
2. Secondary Groups: Listed in /etc/group as comma-separated usernames

For comprehensive checks, use these commands:

# Check primary group (shows GID)
id filesender_1

# Check all groups (including primary)
groups filesender_1

# Alternative verbose output
id -a filesender_1

To make filesender_1 appear in /etc/group while keeping its primary group:

# Add secondary group membership
usermod -a -G valid_senders filesender_1

# Verify
grep valid_senders /etc/group
# Now shows: valid_senders:x:12345:production_1,filesender_1

• Performance: Primary groups are checked more frequently
• Backward Compatibility: Maintains traditional Unix behavior
• File Creation: New files inherit primary group by default

For system administrators, here's a robust verification script:

#!/bin/bash

check_user_group() {
    local user=$1
    local group=$2
    
    if ! grep -q "^${user}:" /etc/passwd; then
        echo "Error: User $user does not exist"
        return 1
    fi
    
    local gid=$(grep "^${group}:" /etc/group | cut -d: -f3)
    if [ -z "$gid" ]; then
        echo "Error: Group $group does not exist"
        return 1
    fi
    
    local primary_gid=$(id -g $user)
    local all_groups=$(id -G $user)
    
    if [ "$primary_gid" = "$gid" ]; then
        echo "$user has $group as primary group (GID: $gid)"
        return 0
    fi
    
    for grp in $all_groups; do
        if [ "$grp" = "$gid" ]; then
            echo "$user has $group as secondary group"
            return 0
        fi
    done
    
    echo "$user is NOT member of $group"
    return 1
}

# Usage example:
check_user_group filesender_1 valid_senders